When a shielded circular coil with n turns and a radius of a meters is placed upon the surface of a semi-infinite medium having a conductivity of sigma mhos per meter and a permittivity of epsilon farads per meter, the incremental resistance in ohms is given by (1)Delta R equals 19.92 F2M a3 n2 sigma where FM is the frequency in megahertz. Similarly, the incremental reactance in ohms is (2) Delta equals 1.25.10 to the 8th power F3M a3 n2 epsilon. Since Delta R and Delta X can be calculated from bridge, impedance meter, or other measurements made at the sending end of a transmission line feeding the coil, (1) and (2) offer a relatively straightforward method for the in vivo evaluation of sigma and epsilon for an individual organ at the time of surgery without invading the organ. Preliminary calculations suggest that this method may be applicable over the frequency range of a few megahertz through a few 10's of megahertz. For (1) and (2) to apply directly with acceptable accuracy to finite sized organs, the coil must be small enough so that the linear dimensions of the organ are several times the diameter of the coil. We intend to design and fabricate simple coil probes, verify that (1) and (2) are valid over the predicted frequency ranges by evaluating in vitro samples with both probe and conventional methods, and then to use our probes for in vivo measurements in dogs to find sigma and l epsilon as functions of frequency for appropriate organs and for different types of tissue. Our data should (a) indicate whether published data derived from earlier in vitro studies are applicable to the in vivo situation and (b) be of value to those involved in the design of systems for the transmission of radio-frequency energy or signals into or out of the body. Furthermore, because the proposed system is very simple and presumably capable of easily yielding reproducible results, it might eventually be of use in clinical studies to find whether sigma and epsilon change in various diseased states.